Band pass filter circuit



n RU T..C w M T Hn CF WP Em A B Dec. 1, 1953 2 Sheets-Sheet l Filed Sept. 29, 1948 BANDWIDTH UENCY FI 5 FREQ INVENTOR. FRED. w. SCHMIDT, JR.

BY W WQA ATTORNEY Dec. 1, 1953 F. w. SCHMIDT, JR

BAND PASS FILTER CIRCUIT 2 Sheets-Sheet 2 Filed Sept. 29, 1948 FIG. 5

INVENTOR. FRED W. SCHMIDT, JR.

WWW ATTORNEYS Patented Dec. 1, 1953 UNITED PTENT OFFICE BAND PASS FILTER CIRCUIT Application September 29, 1948, Serial No. 51,728

My invention relates to electrical circuits for passing a limited band of frequencies, commonly known to the art as band pass filters, and more specifically to those types of pass band filters where the band of passed frequencies may be tuned, or shifted through a portion of the frequency spectrum.

An object of my invention is to provide a tunable band pass filter having a bandwidth more constant over a wide band of tuned frequencies than those of the prior art.

Another object of my invention is to provide a tunable band pass filter having an impedance more constant with tuning than those of the prior art, so that when used in an amplifying circuit, constant gain over the tuning range is readily obtained.

A. third object of my invention is to provide a circuit adapted to a high frequency wide band application. such as the input circuits of a television receiver, whose elements are physically realizable, readily available, and relatively inexpensive.

A fourth object of my invention is to provide a tunabl band pass filter readily adaptable to the inclusion of a circuit for rejecting a fixed frequency outside of the pass band. In the case of a superheterodyne receiver this can be used as an intermediate frequency trap.

In order to explain the operation and advantages of my invention, I shall refer to the accompanying diagrams in which:

Fig. 1 is a band pass filter known to the art; Fig. 2 is a filter modified to embody my invention; Fig. 3 illustrates bandwidth vs. frequency characteristics of pass filter, showing the bandwidth obtainable as a function of tuned center frequency, using several varities of tunable band pass filters; Figs. 4 and 5 are television amplifier circuits containing embodiments of my invention.

Referring to Fig. 1, a primary tuned circuit comprises a source of voltage l at controllable frequency in series with a capacitor 2, a variable inductor 3, and a coupling element, which coupling element in this case is a fixed inductance l. Across the terminals of the coupling element 4 is a series combination of a second variable inductor E5 and a capacitor 6, This circuit is a well known band pass filter circuit, which has been thoroughly analyzed by C. B. Aiken, in his article Two mesh tuned coupled circuit filters, Proceedings of the Institute of Radio Engineers, February 1937, and others, and can be tuned over a range of frequencies by means of the variable in-' ductors 3 and 5.

7 Claims. (01. 333-70) In Fig. 2, illustrating my invention, the coupling element t has been replaced by a coupling network 5, containing a series connection of capacitance 8 and inductance 9, in parallel with an inductance it. Capacitance 6 and inductance 5 are so selected that series resonance occurs at a frequency higher than the tunable range of the filter. The inductance It in combination with the series elements, capacitance 8 and inductance 9, provides parallel resonance at a frequenc below the tunable range of the filter. In some physical embodiments of my invention there can be incorporated into the coupling capacitor 8 and leads enough inductance so that the additional series inductance element 9 is unnecessary.

In Fig. 3 ar shown bandwidths obtained, using several types of coupling elements. From the circuit analysis, in the case of circuits coupled more than transitionally, bandwidth is approximately proportional to the product of midband fre-' quency, hereinafter referred to as frequency, and coupling factor, which latter is the ratio of coupled reactance to total series reactance of the tuned circuits.

In the inductively coupled circuit of Fig. l, the impedance of the coupling element increases directly with frequency. Since tuning is by means of variable inductance elements, the total reactance of each tuned circuit varies inversely with the frequency. Hence, the coupling factor increases with the square of the frequency, and the bandwidth increases approximately as the third power of the frequency, as shown in curve I! of Fig. 3.

If the coupling inductance l be replaced by a capacitive coupling element, which normally varies in reactance inversely with frequency, the coupling coeflicient will be constant with frequency, and bandwidth will be approximately proportional to frequency. This is shown in curve [2.

Using the coupling network of my invention, which contains both capacitance and inductance, there is obtained. a bandwidth considerably more constant with frequency than that obtained using inductance or capacitance alone. This is shown in curve It of Fig. 3. This result appears even more surprising when it is noticed that over certain ranges at high and low ends of the frequency range, bandwidth decreases with increasing frequency, although in the case of inductive or capacitive coupling alone, bandwidth increases with frequency throughout the entire range.

The reason for this is believed to be as follows: If the frequency range is reasonably wide,

3% the effects of the coupling inductances 9 and ID are not great at frequencies in the center of the band, and bandwidth can increase with frequency as in the case of capacitive coupling. At the high frequency end of the band the effect of series inductance 9 becomes pronounced, causing the reactance of the. network to, be much less than that of the inductance lil alone. This in turn, reduces bandwidth. The shape of the bandwidth curve at the high end can be controlled by the value of the series inductance 9 but. will always have less variation than the curve of capacitive coupling, as shown in the high-frequency end. of.

Fig. 3. At the low frequency: end-,the bandwidth. curve can be raised by the use of the parallel inductance H), which increases the reactance. of the network beyond that of the capacitance 8 alone, thus increasing the bandwidth. By use, of either the series inductance 9 or the parallel inductance it, but not. both, in combination with capacitance 8', unsymmetrical compensation may be obtained, producing bandwidths more uniform than that: of either inductive or capacitive, C0117 pling: alone, but falling short of the results obtained here using the preferred network.

In Fig. there is shown a television receiver input circuit using this invention. A tube l4 provides a; signal, tlns being one of the amplifier tubes in a television receiver. A suitable source id of positive potential is connected to the anode of the tube l4, through a suitable load resistor It. This load resistor l6 also provides damping resistance: for the primaryv tuned circuit. The tube i4 is coupled to a first variable inductance element 3. through a small blocking capacitor H. The coupling network connected inv accordance. with this invention is illustrated by the reference numeral 7 and comprises. the elements as described above-of a capacitance 8 in series with an inductance, 9, this series connection being inparallel with an inductance ill. The coupling net.- worlr. l is coupled to a second tube. l in the television receiver through a second variable induct,- ance element 5. and a second blocking capacitor it; A source. t9, of negative potential is; connected through a resistor 26 to the grid of. the.

mixer tube 2! to provide bias volta e therefor. This grid resistor 26 also provides damping for the secondary circuit. Thelumped capacitance of; thetube, the elements and the wiring, etc, is illustrated in dotted lines by thev reference numeral 22 and constitutes the. primary tuned capacitance corresponding to the capacitance 2 illustrated in Fig. 2. Similarly, the lumped distributed capacitance. of the secondary circuit including the input of the tube 2|. is illustrated. in dotted linesby the reference numeral 23 and.- corresponds. to the capacitance 6 of Fig. 2. A. local oscillator circuit 24 iscoupledto. the grid of. tube 2| by means of a small capacitance 25.

Fig, 5, is. a. circuit. similar to that. of Fig. 4, except .thata capacitor 2.6 has beenadded. in series with the low frequency compensating inductance Hi. The value of the capacitor 26 preferably is so chosen that it will resonate. withthe inductance. I I). in seriestherewith substantially atthe intermediatefrequency. of the receiver. This new element ZG-has very little effect upon. the operation of, the filter withinthe pass band, but provides a convenient trap to prevent the passage of intermediate frequencies through the filter. Since the coupling network I has no direct current path to ground in this illustration, the blocking condenser [8 of Fig, 4 isunneeessary and has been omitted;

In. the preferred form of; the invention. the

tuning inductances 3 and 5 are ganged together to be substantially equal to one another over the tuning range. Also primary and secondary circuits are tuned to the same frequency and are substantially equally damped. These conditions are by no means necessary to the successful operation of the; invention, and if certain physical factors, such as damping, could be economically controlled, certain minor advantages might resuit from unequal damping or stagger tuning.

From the conditions that exist in the preferred form of my invention, namely, inductance tuning; and shunt damping of the tuned circuits, another advantage. follows from the theory, and is actually very'welli realized in practice; namely, constant. input impedance over a wide tuning range. When applied to an amplifier circuit, there resultsthe very desirable condition of constant gain, signal-to-noise ratio, and time delay Within the band. Outside. of the band, because. of the same factors, image, and adjacent channel rejection. are.v also. constant, which is also desirable.

Although the preferred form of invention shows two. tuned circuits, coupled together, the invention applies to circuits containing three or more.

tuned circuits, and the desirable. results of more constant bandwidth and impedance will be obtained. Similarly other modificationswill be apparent to those skilled in, the art without departing from the scope of the. invention.

What is claimed is:

1'. In. a tuning circuit tunable. over a range of frequencies and having a substantially fixed. bandwidthv of. response, the combination. of a.

first tuned circuit, containing inductive and capacitive reactance elements connected in series.

a second tuned circuit. containin inductive and capacitive reactance. elements connected in series.

one of said. reactance. elements in each said circuit being variable, each said. circuit being tunable over a predetermined range. of frequencies and a coupling network connected jointly to'said.

tuned circuits and providing impedance common to. said first tuned circuit andsaid. second tuned.

circuit; said coupling network containing inductance and capacitance series resonant at a frequency' higher than the tunable range of said first tuned circuit and said second. tuned circuit, and parallel resonant at a frequency lower than said tunable range. I

2'. Ina tuning circuit tunable over a predetermined range of frequencies, and having a substantiallyfixed bandwidth of response the combination of a variable inductance, a capacitance,

and a coupling network, said combination being connected in series electrically, saidcoupling net- Work havingacross its terminals a series com-- bination of a second variable inductance and a second capacitor, said coupling network contain- 111% inductance and capacitance series resonant at a frequency beyond one end of, the tunable la1;*r:1ts-of'said tuningcircuit, and parallel resona'a fre ucnc Y I t v tunable. limit 1S. J beyond theother end of said 3. Ina tuning circuit tunable over a predetermined range of frequencies, and havinn a substant ally fixed" bandwidth of response the combination of: a variableinductance, a capacitan e and a coupling network, said combination bein connected in series electrically, said coupling net work having across its terminals a series com blnation of a second variable inductance and a second capacitor; said coupling network comprismg; aseriescombination of coupling inductance and a coupling capacitor-resonant at a fre uer higher than tne tunable range of said ton circuit, and a parallel combination of a coupling inductance and coupling capacitance resonant at a frequency lower than said tunable range.

4. In a tuning circuit tunable over a predetermined range of frequencies, and having a substantially fixed bandwidth of response the com bination of a first variable inductance, a first capacitance, and a coupling network, said combination being connected in series electrically, said coupling network having across its terminals a series combination of a second variable inductance and a second capacitance, said coupling network comprising a couplin inductance in parallel with a series combination of a coupling capacitance and a second coupling inductance, said coupling network being series resonant at a frequency higher than the tuning range of said tuning circuit, and parallel resonant at a frequency lower than the tuning range of said tuning circuit.

5. A tuning circuit tunable over a predetermined range of frequencies and having a substantially fixed bandwidth of response, comprising two tunable resonant circuits each having inductive and capactive reactance elements connected in series, one of said reactance elements in each of said circuits being variable, and a twoterminal coupling network having one terminal connected jointly to said tunable circuits to provide a common electrical path for currents in said tunable circuits, said coupling network being parallel resonant below and series resonant above the tuning range of said tuning circuit.

6. In a superheterodyne receiver having intermediate frequency stages, a tuning circuit tunable over a predetermined range of frequencies and having a substantially fixed bandwidth of response and comprising a plurality of two-terminal tunable resonant circuits, and a two-terminal coupling network having one terminal connected jointly to one terminal of each of said resonant circuits to provide a common electrical path for currents in said resonant circuits, said coupling network being series resonant at a frequency above said predetermined range, parallel resonant at a frequency below said predetermined range, and series resonant at said intermediate frequency.

7. In a superheterodyne receiver having intermediate frequency stages, a band pass filter tunable over a predetermined range of frequencies and comprising a plurality of two-terminal tunable resonant circuits, and a two-terminal coupling network having one terminal connected jointly to one terminal of each of said resonant circuits, said coupling network comprising a first series combination of capacitance and inductance connected in parallel with a second series combination of capacitance and inductance, said network being series resonant at a frequency above said predetermined range, parallel resonant at a frequency below said predetermined range, and series resonant at said intermediate frequency.

FRED W. SCHMIDT, JR.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,227,113 Campbell May 22, 1917 1,568,144 Elsasser Jan. 5, 1926 1,644,004 Zobel Oct. 4, 1927 1,800,996 Gerth Apr. 14, 1931 1,350,931 Elliott Mar. 22, 1932 2,001,090 Bode May 14, 1935 2,052,338 Budenbom Aug. 25, 1936 2,167,079 Landon July 25, 1939 2,206,388 Buschbeck July 2, 1940 2,250,519 Beers July 29, 1941 2,471,705 Schmitt May 31, 1949 

